The present invention relates to nickel positive electrode plates for alkaline storage batteries like nickel-cadmium storage battery and nickel-metal hydride storage battery and a method for producing the same.
With growing uses of alkaline storage batteries, there is an increasing need for weight reduction, size reduction and improvement in performance of those batteries. Especially needed are nickel positive electrode plates for use in electric vehicle batteries that can be charged and discharged in a short time and which is high in utilization of active material at a high temperature and has a long cycle life.
The positive electrode plate that has been used for rapid charging alkaline storage batteries is generally a sintered-type nickel electrode that is low in internal resistance and excellent in cycle characteristics. This sintered type nickel electrode is mixed with cadmium or cobalt to improve the utilization of active material at a high temperature.
Recently, however, regulations are getting tightened on the use of cadmium compounds as awareness of the environmental preservation is rising. Furthermore, the nickel-metal hydride storage battery in which the negative electrode comprising a hydrogen storage alloy as main material is used is known to have this problem. That is, the cadmium compound added to the positive electrode migrates and covers the surface of the negative electrode in charge and discharge cycles, hindering the working of the negative electrode in charge and discharge.
In making a positive electrode with a cobalt compound, meanwhile, a large quantity of expensive cobalt has to be added to secure sufficient characteristics. Another problem is that the addition of cobalt in a large quantity causes the voltage to drop during discharging.
A solution to those problems was disclosed in Japanese Laid-Open Patent Publication No. Sho 48-50233, for example. The technique includes adding an yttrium compound such as yttrium hydroxide to the nickel positive electrode to raise the utilization of active material at a high temperature without causing the utilization of active material at room temperature to drop. The positive electrode with yttrium hydroxide added thereto can be prepared as follows, for example. Sintered nickel substrate is dipped in an acidic aqueous solution containing a nickel salt and an yttrium salt, and dried, and then dipped in an alkaline aqueous solution to convert the nickel salt and yttrium salt into hydroxides. This series of treatments is repeated until a desired amount of active material is produced.
The problem with the technique is that the amount of yttrium increases and that because nickel hydroxide and yttrium hydroxide are formed in a mixed state, the apparent specific gravity is low and it is impossible to obtain an electrode with a high capacity density.
It is an object of the present invention to provide a sintered-type nickel positive electrode plate for alkaline storage batteries with a high capacity density and a high utilization at a high temperature in which the drop in working voltage during discharging is kept down by using the yttrium compound and cobalt compound in appropriate quantities.
The present invention provides a nickel positive electrode comprising a porous sintered nickel substrate and metal hydroxides mainly composed of nickel hydroxide impregnated in the substrate, the metal hydroxides including: a first layer comprising a hydroxide of at least one element selected from the group consisting of yttrium, ytterbium and lutetium, and being in contact with the substrate; a second layer comprising a mixture of nickel hydroxide and cobalt hydroxide, and covering the first layer; and a third layer comprising cobalt hydroxide and covering the second layer.
In a preferred mode of the present invention, the hydroxide in the first layer of at least one element selected from the group consisting of yttrium, ytterbium and lutetium is 1 to 4 parts by weight, the amount of cobalt hydroxide in the second layer is 1 to 5 parts by weight, and the amount of cobalt hydroxide in the third layer is 3 to 6 parts by weight, each per 100 parts by weight of the nickel hydroxide.
In another preferred mode of the present invention, the second layer further comprises zinc hydroxide. It is preferable that the amount of zinc hydroxide is 3 to 6 parts by weight per 100 parts by weight of the nickel hydroxide.
The present invention provides a method for producing a nickel electrode plate comprising the steps of:
(1) forming a first layer comprising a hydroxide of at least one element selected from the group consisting of yttrium, ytterbium and lutetium in pores of a porous sintered nickel substrate by conducting at least once a procedure including dipping the substrate in an acidic aqueous solution of a salt of at least one element selected from the group consisting of yttrium, ytterbium and lutetium, followed by drying and dipping the same in an alkaline aqueous solution to convert the salt into a hydroxide,
(2) forming a second layer comprising a mixture of nickel hydroxide and cobalt hydroxide on the first layer of the substrate by conducting at least once a procedure including dipping the substrate in an acidic aqueous solution containing a nickel salt and a cobalt salt, followed by drying and dipping the same in an alkaline aqueous solution to convert the nickel salt and cobalt salt into nickel hydroxide and cobalt hydroxide, respectively, and
(3) forming a third layer comprising cobalt hydroxide on the second layer by conducting at least once a procedure including dipping the substrate in an aqueous solution of a cobalt salt, followed by drying and dipping the same in an alkaline aqueous solution to convert the cobalt salt into cobalt hydroxide.
The present invention further provides a method for producing a nickel electrode plate comprising the steps of:
(1) forming a first layer comprising a hydroxide of at least one element selected from the group consisting of yttrium, ytterbium and lutetium in pores of a porous sintered nickel substrate by conducting at least once a procedure including dipping the substrate in an acidic aqueous solution of a salt of at least one element selected from the group consisting of yttrium, ytterbium and lutetium, followed by drying and dipping the same in an alkaline aqueous solution to convert the salt into yttrium hydroxide,
(2) forming a second layer comprising a mixture of nickel hydroxide, cobalt hydroxide and zinc hydroxide on the first layer of the substrate by conducting at least once a procedure including dipping the substrate in an acidic aqueous solution containing a nickel salt, a cobalt salt and a zinc salt, followed by drying and dipping the same in an alkaline aqueous solution to convert the nickel salt, cobalt salt and zinc salt into nickel hydroxide, cobalt hydroxide and zinc hydroxide, respectively, and
(3) forming a third layer comprising cobalt hydroxide on the second layer by conducting at least once a procedure including dipping the substrate in an aqueous solution of a cobalt salt, followed by drying and dipping the same in an alkaline aqueous solution to convert the cobalt salt into cobalt hydroxide.